Rode a bike down a staircase and killed a goon by pinning him against the wall, and trapping his friend behind the bike at the same time. Then I hit his friend so hard he did the splits while standing up.

If you go to your map and press X, or square depending on your platform of choice, you can see all available collectibles (according to which girlfriend missions you have completed). The ones you have collected are white, whilst the ones you haven't are dulled and grey.